1. Field of the Invention
The present invention relates to a steering angle sensor detecting a rotation angle of a steering mounted on a vehicle, and particularly, to a technology for improving accuracy of the detection.
2. Description of the Related Art
As a conventional example of a steering angle sensor mounted on a vehicle, there is one using a system, in which a large diameter gear is disposed coaxially to a steering, and further, a small diameter gear meshing with this large diameter gear is disposed, and a rotation angle of this small diameter gear is detected.
As shown in FIG. 1, accompanied with the steering shaft 101 rotates, the large diameter gear 102 rotates, and the small diameter gear 103 meshing therewith is rotationally driven. In addition, the magnet 104 is disposed on the center of the small diameter gear 103, and further, the hall IC 105 is disposed in the vicinity of where the magnet 104 is disposed. Hence, the direction of the magnet 104 can be detected by the hall IC 105, and eventually, the rotation angle of the small diameter gear 103 can be detected.
Accordingly, the rotation angle of the steering shaft 101 can be calculated based on output signals of the hall IC 105.
However, in the steering angle sensor thus configured, the number of teeth of the small diameter gear 102 is smaller than that of the large diameter gear 102, in which, for example, a ratio of the numbers is 1:4. Moreover, in the steering shaft 101, the total steering angle is approximately less than 4 turns, and therefore, the small diameter gear 103 will make a total of approximately 15 turns.
Hence, though the hall IC 105 can detect the rotation angle of the small diameter gear 103, the hall IC 105 cannot recognize the absolute position of the steering shaft 101, that is, cannot recognize how many turns the small diameter gear 103 has made among the total of 15 turns. Therefore, the conventional steering angle sensor has had a drawback that the sensor must detect the steering angle with reference to a steering angle of the steering shaft 101 of 0 degree, that is, with reference to a straight ahead position of the vehicle, which makes it impossible to detect the steering angle until a reference position signal to notify that the vehicle is at the straight ahead position is given from the vehicle.
Meanwhile, in order to solve this problem, there has been proposed a method of constantly monitoring variations of the steering angle by energizing the steering angle sensor even when the ignition switch of the vehicle is turned off. However, this method has caused a problem that a heavy load is put on a battery because electric power is consumed even when the ignition switch is turned off.
As described above, the conventional steering angle sensor has had the problem that the sensor cannot detect the steering angle until the straight ahead position of the vehicle is detected. Furthermore, the method in which the steering angle sensor is energized even when the ignition switch is turned off has had a drawback that the method was impractical because more electric power is consumed.
The present invention has been created in order to solve the conventional problem as described above. It is an object of the present invention to provide a steering angle sensor capable of detecting a steering angle of a vehicle by a simple method.
In order to achieve the foregoing object, the present invention provides a steering angle sensor, including a first gear rotating in synchronization with a steering shaft, a second gear rotating in synchronization with the first gear at a rotation speed faster than a rotation speed of the first gear, a small angle detection magnet provided on the second gear and rotating together with the second gear, a small angle detection magnetic sensor disposed in a vicinity of where the second gear is fixed and configured to detect a magnetic line of force of the small angle detection magnet, a third gear rotating at a rotation speed slower than the rotation speed of the second gear in synchronization with the first gear, a large angle detection magnet provided on the third gear and rotating together with the third gear, and a large angle detection magnetic sensor disposed in a vicinity of where the third gear is fixed and configured to detect a magnetic line of force of the large angle detection magnet, wherein a rotation angle of the steering shaft is calculated based on angle data detected in the small angle detection magnetic sensor and the large angle detection magnetic sensor.
The steering angle sensor may be configured such that the small angle detection magnetic sensor supplies a periodic signal of a triangular wave accompanied by a rotational movement of the second gear, and that the large angle detection magnetic sensor supplies a periodic signal of a triangular wave accompanied by a rotational movement of the third gear, in which the periodic signal has a longer period than a period of the periodic signal supplied by the small angle detection magnetic sensor.
The steering angle sensor may be configured such that detection data by the small angle detection magnetic sensor and detection data by the large angle detection magnetic sensor are set different from each other in accordance with an absolute angular position of the steering shaft.
The steering angle sensor may be configured such that a signal, in which a gradient of a detection value detected by the large angle detection magnetic sensor is made to coincide with a gradient of a detection value detected by the small angle detection magnetic sensor, is created as a conversion signal, and that a difference between the conversion signal and a detection result by the small angle detection magnetic sensor is calculated, and it is determined as to which period in a periodic waveform supplied from the large angle detection magnetic sensor an absolute steering angle of the steering shaft belongs in accordance with the remainder when a value of the difference is divided by a predetermined number of steps.